1. Technical Field of the Invention
This invention is related to an improved motor structure, in particular, a stepping motor structure having an improved magnetic circuit.
2. Description of the Prior Art
Stepping motors are widely used in equipment for office automation, computer peripheral devices, industrial apparatus, etc. The stepping motor is an electric motor to be rotated by input pulse, and its rotation angle is proportional to input pulse number and its rotational speed is proportional to the frequency thereof. Accordingly, it is unnecessary to detect the rotor position in the stepping motor. That is, since the stepping motor is driven while forming open loop, the rotor is rotated while minimizing the magnetic resistance. Accordingly, the rotational state of the rotor of the stepping motor depends on the magnetic flux generated from the rotor, the armature magnetic flux generated from the stator of the motor, and the rotor inertia.
Next, since the stator pole teeth of the stepping motor, especially the stator pole teeth of the claw pole type stepping motor, is formed by punching and folding a plate of soft magnetic material it is impossible in respect of manufacturing technology to prepare very long stator pole teeth. Nevertheless, when it is still desired to prepare long stator pole teeth, generally, the dedenda of the pole teeth are widened to be in the form of trapezoid so as not to produce magnetic saturation of the pole teeth, and the axial lengths of the rotor magnets facing the pole teeth are made to be equal to or longer than the axial lengths of the pole teeth such that the magnets are superimposed on the pole teeth when it is projected thereon.
Furthermore, two kinds of the stator pole teeth constituting one phase are generally formed into the same shape. Accordingly, when driving the stepping motor, changes of the rotational state are caused in conformity with the frequency variation of the input pulse, and the stepping motor is subjected to vibration, thereby bringing about such problems as the exertion of an undesirable influence on the other components of the recent high performance equipment and the high possibility of causing troubles with the equipment. A means for suppressing the vibration of the stepping motor is presented in Japanese Patent Application No. H8-276487, or the like. However, such a method is to reduce the vibration by changing the area where the rotor magnet and the side edge of the pole teeth face each other, and thereby the rotor magnet has its length sufficient to cover the whole lengths of the pole teeth constituting one phase. Accordingly, it is difficult to minimize the magnet length for resource saving, and further downing, lightening and cost reduction of the motor cannot be achieved.
The invention has been made in the light of the above. One object of the invention is to provide a new motor structure where there is removed an immediate cause for generating the vibration disadvantageous to a motor such as a stepping motor to thereby restrain the vibration effectively. Another object of the invention is to provide a low manufacturing cost motor with little vibration.
An effective approach to lowering vibration of the stepping motor is to decrease the detent torque. Then, the presently available method therefor is to wide the clearance between the rotor and the pole teeth, which can surely lower the detent torque. However, it leads to decrease the dynamic torque and thereby deteriorate performance of the motor.